ESR signals from clusters of iron ions as indicators of the thermal history of fault gouge

Samples taken from four different faults at a depth of 400 m show no sign of a bulk heating higher than about 350°C (heating time 15 min) during fault activities. Besides radiation defects, the aggregation behavior of iron ions studied by electron spin resonance (ESR) can be used as a measure for the thermal history of fault gouge material. Clusters of iron ions are formed as a result of the heating of biotite and feldspar minerals. They give broad (ΔB ≈ 200 mT) and intense signals in the ESR spectra. Mn2+ point defects react very sensitively to crushing of the samples, while relatively isolated and coupled Fe3+ point defects in the feldspars do not.